1. Field of the Invention
The invention relates to a device for filtering a liquefied plastic, with a housing, a supply channel, a discharge channel and backflush channels, whereby in the flow path of the plastic melt, in two screen carriers slideably supported crosswise to the flow direction, at least one filter element each is arranged in a corresponding screen chamber and can be linked to the supply channel and the disposal channel, and a displacement piston that loads the clean screen side with purified plastic melt when the screen carrier is in the backflush position.
2. Discussion of Background Information
A filtration device for contaminated fluids and method of operation is known from EP 0 554 237 A. In this known device at least one reservoir for holding a quantity of fluid required in the backflushing operation is connected to at least one discharge channel. A piston for displacing fluid out of the reservoir is guided in the reservoir, whereby during the backflushing operation this reservoir is in free flow connection for the purified fluid with the discharge side of the screen nest to be backflushed and with the discharge channel. During normal production the reservoir or storage chamber is filled with plastic material and, in the case of backflushing, is not only connected to the screen nest to be backflushed, but the displacement piston guided in the reservoir also acts on the normal production channel that leads from the screen then in operation to the equipment or tool. The problem occurs here that, with the reservoir opening into the production channel, the plastic mass to be forced out of the reservoir in part has to flow against the flow direction in the production channel in order to backflush the screen. However, part of the plastic material forced out of the reservoir also flows in the direction of the production channel, i.e., in the direction towards the tool. Problems therefore occur here that are difficult to overcome in practice.
The pressure constant mentioned in this reference cannot be achieved, since it is here a matter of communicating tubes, i.e., connected pressure chambers. Furthermore, the resistance towards the backflush opening is much lower than towards the tool, so that a substantial pressure loss must occur at the tool itself.
WO 98/47688 describes a screen unit for viscous masses, i.e., liquefied plastic, in which likewise a displacement piston forces a purified plastic mass back through the screen in order to thus carry out the backflushing operation. This arrangement has the disadvantage that valve devices have to be present that close the backflush channel during the normal mode of operation, thus during the production phase. Such valve devices are undesirable in the processing of fluid thermoplastic material, since they can lead to clogs and, naturally, in addition must be heated. A decomposition of the plastic material can also occur in this area, since the valve device can never be cleaned completely.
Furthermore this known device has the disadvantage that the entire valve device has to be removed to replace the screens, i.e., the known device is therefore embodied in a relatively complex manner.
A device is known from DE 17 88 037 U1 for separating mixtures containing liquid and solid matter, in which the liquid containing solid material is guided through a screen, whereby an accumulator piston is moved back at the same time. A discharge line is connected to the area of the accumulator piston, through which line the purified liquid is discharged. The connection between the accumulator piston and the discharge line is equipped with a check valve that opens when the accumulator piston generates a corresponding pressure in the discharge line. At the same time, the purified liquid can also be pressed through the screen again with the movement of the accumulator piston, in order to clean the screen of trapped solid particles. A device of this type cannot be used for liquefied plastic, in particular because in the discharge channel the check valve provided there would very quickly clog and become unusable. Moreover, material being retained in the area between the check valve and accumulator piston would disintegrate, i.e., dead zones are also present here which definitely must be avoided in processing liquid plastic.